A load/unload control scheme may be used to control a large variety of applications. A load/unload control is well suited for arrangements where a compressor is connected to a large pressure vessel, for example. Under the load/unload control scheme, a compressor operated at a constant speed is controlled to a load mode or an unload mode in turn.
In the load mode, the compressor generates mass flow, thus increasing the pressure in the pressure vessel. When the pressure in the pressure vessel reaches a set maximum limit, the compressor is set to the unload mode.
In the unload mode, the compressor still runs, but generates does not generate mass flow. Various strategies may be used for unloading a compressor. For example, a compressor may be controlled to a load mode or an unload mode by controlling an inlet valve of the compressor. As no mass flow is generated, the pressure in the pressure vessel starts to decrease. In the unload mode, the compressor may only use a fraction of the power consumption during the load mode. The rate of decrease may depend on the output demand. When the pressure drops to a set minimum level, the load mode is reactivated.
In order to determine when to switch to the load mode or to the unload mode, the compressor system may comprise a pressure sensor or a sensor for detecting when a limit has been reached. Such sensors may increase the cost of the compressor system. Further, the sensors may be prone to malfunctions and may require regular maintenance.